1. Field of the Invention
The present invention relates to a lamination for electric motors and a method for making this lamination and coupling more laminations to each other.
2. Discussion of Related Art
The stators and rotors of several types of electric motors are made by packing a plurality of suitably shaped laminations made of ferromagnetic material. The individual laminations are generally obtained from metal laminates that are subjected to pressing and punching processes.
The thus-obtained laminations are coupled to each other; in particular, they are stacked to form the core of a rotor or to form a stator. Each lamination is provided with slots, which along with the slots of the other laminations define the slots suitable to house the stator/rotor windings or the melt material alternatively used (generally die-cast aluminium).
The laminations used for making rotors of electric motors can be coupled such that the rotor has straight or skew slots, such as having a helical development. In other words, the laminations can be stacked on each other without offset, such that the slots for the windings are overlapped to form a straight slot, or with an angular offset, such that the slots of a first lamination result to be rotated relative to the matching slots of a second lamination adjacent thereto, in order to form a slot for the winding which is either skew or helical.
The laminations are coupled to form a pack having the desired height, corresponding to the height of the rotor or stator of the electric motor to be made. Regardless of the slot shape, when the pack is made up of a large number of laminations, any difference in the thickness that can be found between the different portions of the laminations can lead to inaccurate assembly. For those packs made up of a large number of laminations, for example more than 100, a “compensation” may be required during the manufacturing step. The compensation is carried out by stacking the laminations such that the pack mass is evenly distributed relative to the axis thereof. For example, the rotors or stators are “compensated” by packing each lamination offset by a preset angle, such as 90° relative to the adjoining lamination (and this is provided for all the laminations in the pack) such that any non-uniformity of the individual lamination is evenly distributed relative to the axis of rotation of the lamination pack (rotor axis).
The coupling between the laminations is generally obtained by providing each lamination with one or more bosses, which can be, in turn, provided with a relief hole. The bosses are projections of the lamination, which are obtained during the pressing step, which branch off from one of the two faces thereof and develop along an arc of circle between two ends. The (through) relief holes, which are also obtained by means of punching, are formed at an end of the relative boss.
In a lamination for stators, the bosses are not provided with a relief hole and the ends of each bosses are integral with the lamination. The bosses have the function of engaging the corresponding bosses of a second adjoining lamination.
In a lamination for rotors, each boss is provided with a matching relief hole. At this hole, the end of the boss is separated from the lamination and remains free. The relief hole allows creating a sufficient space for inserting the free end of a matching boss of a second lamination, when the laminations are stacked in an offset manner in order to obtain skew slots. The laminations being at the one end of the pack, i.e. the first one, or the first laminations in the pack, are usually not provided with bosses and are called the separating laminations.
The coupling between the matching bosses of adjoining laminations is of a forced type (at the side edges of the bosses). The drawbacks of the traditional laminations are generally connected with the boss-boss coupling, particularly when the laminations are coupled in an offset manner. As the height of the lamination pack increases, i.e. when the number of coupled laminations increases, the offset angle between two adjoining laminations is small. The interference that is likely to occur between the ends of the coupled bosses can affect the evenness of the offset angle along the pack. In other words, the bosses being coupled can interfere at the ends, thus actually changing the preset offset angle.
This problem is particularly important when the laminations are coupled in order to make rotors with skew or helical slots. The offset angle between adjoining laminations has to be even along the rotor as much as possible. On the other hand, the least variation in this angle leads to slots having a development in the space that deviates from the desired trend. The filling material of the rotor slots, such as aluminium, is arranged in the relative slot by following the development thereof. The displacements of the slots, and thus of the aluminium cage housed therein, from the desired skew or predetermined helical development cause the variation of the electrical characteristics of the relative electric motor.
In other words, in packs consisting of a large number of laminations, which are used to provide long rotors with skew or helical slots, laminations with an even offset angle are particularly difficult to couple. As the number of laminations increases in the pack, the offset angle between two adjoining laminations must be reduced. When an offset angle of a few degrees or fractions of degree must be adopted, any interference between the ends of the coupled bosses can easily cause sensible displacements of the offset angle from the desired value.
The manufacturers of laminations for electric motors and machines for manufacturing the same have been attempting for some time to develop a method for providing laminations that can be more precisely packed than traditional laminations.
Alternatively to the method described above for coupling laminations, a system has been proposed which provides using bosses and coupling holes.
For example, the European Patent Application EP-A-0847109, in the name of CORRADA S.p.A., describes a lamination provided with bosses and coupling holes being arranged along the circumference thereof. The distance between the bosses and holes is selected such that the laminations can be stacked by providing an offset angle between each lamination. Thereby, besides being able of providing rotors with both straight and skew slots, compensation also requires to be carried out.
A drawback with the lamination provided according to the European Patent Application EP-A-0847109 is the requirement of making the coupling holes for the bosses. With this processing, one is forced to carried out the compensation of the pack. Furthermore, because of the requirement of making the coupling holes, the ferromagnetic material is reduced in the rotor made with the laminations.
It would be desirable is to provide a lamination for electric motors and a method for making the same, which overcomes the drawbacks of the traditional laminations in a simple and effective manner.
It would also be desirable to provide a lamination for electric motors, which can be easily packed, though it is not provided with coupling holes or boss relief holes.
It would be further desirable to provide a lamination for electric motors, which can be easily packed with the maximum precision of positioning and maximum stability.
It would be desirable to provide a method for making laminations for rotors of electric motors with skew (or helical) slots, which are easy to couple with the maximum precision and effectiveness.